The present invention relates to a strobe light control device, which detects the strobe light reflected by an object and control the emission of the strobe light. In particular, the invention relates to a strobe light control device which enables an appropriate exposure regardless of the difference of reflectivity of various objects.
Recently, as a strobe device for a camera, a so-called auto-strobe device is employed. The auto-strobe device is configured to detect strobe light reflected by an object, and to control the strobe to stop light emission when the light amount received from the object has reached a predetermined amount. With this control, the object is exposed to light appropriately.
In such an auto-strobe device, however, the reflectivity of an object is generally assumed to be 18% and the exposure parameter is determined on this assumption. Therefore, regarding a whitish object whose reflectivity is greater than 18%, the photometry value of the reflected light is relatively great and the light emission of the strove is stopped at an earlier stage, which results in an underexposed condition of the object. On the contrary, regarding a dark object whose reflectivity is less than 18%, the photometry value is relatively small, and the emission of the strobe light is stopped later than necessary. In such a case, the object is over-exposed. The difference of the reflectivity of the object occurs not only in a case where the object is whitish or dark, but it may occur for a range of colors of the object. For example, when the color of an object is yellow, the reflectivity may be up to 70%. In such a case, if the standard reflectivity is assumed to be 18%, the exposure value is approximately 2 Ev lower than necessary. If the object color is blue, the reflectivity is approximately 9%. In this case, the object is over exposed by approximately 1 Ev greater than necessary.
Therefore, in the cameras provided with a conventional strobe light control device, the photographer is required to guess the reflectivity of the object. Then, based on the reflectivity determined by the photographer, the exposure using the strobe is controlled such that, if the object is a whitish or yellowish one having a relatively high reflectivity, light emission of the strobe is controlled so that the object is overexposed, and if the object is a blackish or bluish one having a relatively low reflectivity, the light emission of the strobe is controlled so that the object is under-exposed.
With this operation, the above-described defects may be solved. However, accurately guessing the reflectivity of the object and performing compensation of the exposure value can only be done by experienced and skilled photographers. It is impossible to have all the photographers do such an operation. Further, it is not preferable that a manual operation of the photographer is required for exposure. Further, if such a manual operation is required, the camera becomes unsuitable, as the recent trend is for automatic photographing.
It is therefore an object of the invention to provide a strobe control device, which enables an appropriate exposure using the strobe regardless of the difference of reflectivity of the objects.
For the above object, according to the invention, there is provided a strobe control device for a camera, which is provided with a light amount measuring system that measures amount of light emitted by a strobe and reflected by an object, a comparing system that compares the measured light amount with a reference exposure light amount, the reference exposure light amount being determined in accordance with a reference control level, the strobe being controlled to stop emitting light when the measured light amount reaches the reference exposure light amount, a colorimetry system that determines a color of the object, a reflection compensation value determining system that determines a compensation value for compensating for errors depending on a reflectivity of an object, and a compensation system that compensates for the reference control level in accordance with the reflection compensation value.
With this configuration, by adjusting the reference control level (which is referred to as a strobe control level SXVD in the description of the embodiment), which is used as a reference for determining the appropriate exposure amount when the strobe photographing is executed in accordance with the color of the object, regardless of the color of the object and therefore regardless of the reflectivity of the object, the strobe photographing can be performed with appropriate amount of strobe light.
Optionally, the colorimetry system may include a blue photometry sensor for metering a blue light component, a green photometry sensor for metering a green light component, and a red photometry sensor for metering a red light component.
In this case, the colorimetry system may include a light receiving surface divided into a plurality of light receiving areas, and color of the object may be measured in each of the areas. Further, the reflection compensation value determining system may determine the compensation value based on a colorimetry value of the object in said each of the areas.
Further optionally, the strobe control device may include a light emission circuit that drives the strobe to emit light, a detector that outputs an electrical current upon receipt of the light reflected by the object, a TTL integration circuit that integrates the output of the detector, the light emission circuit stops the light emission when an output voltage of the TTL integration circuit reaches a TTL reference voltage, the TTL reference voltage being set corresponding to the appropriate exposure amount, a reference control level being a control level for setting the TTL reference voltage.
In this case, the TTL integration circuit may further include a charging circuit that accumulates electric charges corresponding to the measured light amount and outputs a charged voltage corresponding to the accumulated electric charges, a D/A converter that generates the TTL reference voltage based on a value of the reference control level, and a comparing circuit that compares the charged voltage with the TTL reference voltage, and outputs a quench signal for stopping the light emission of the light emission circuit when the charged voltage exceeds the TTL reference voltage.
Furthermore, the charging circuit may output the charged voltage as a difference with respect to the reference voltage, and wherein the D/A converter generates the TTL reference voltage as a difference with respect to the reference voltage in accordance with the reference control level.
Still optionally, the charging circuit may include a voltage dividing circuit that divides the charged voltage. In this case, one of the charged voltage and a divided voltage which has been divided by the voltage dividing circuit may be selectively applied to the comparing circuit as the charged voltage.
Further, the strobe control device may include a normal light detecting system that performs photometry with respect to the object, a strobe photographing using the strobe control device to be performed being determined based on a measured value of the normal light detecting system.
In a particular case, the camera is a single lens reflex camera provided with a pentagonal prism, and the colorimetric system and the normal light detecting system are arranged on an eyepiece optical system side with respect to the pentagonal prism, the normal light detecting system facing an upper central portion of the pentagonal prism.
Optionally, the green photometry sensor is used as the normal light detecting system, an output of the green photometry sensor being used as the output of the normal light detecting system.